JP2647458C - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] In order to separate a halftone dot region from other regions with high accuracy, an edge density is measured in a local region surrounding a pixel of interest, and the edge density is set to a predetermined threshold value. An edge density detection unit that determines whether or not the target pixel is a halftone dot candidate pixel based on whether the pixel exceeds the target pixel, and compares pixel information of a local region surrounding the target pixel with a predetermined halftone pixel determination pattern. A pattern comparison unit that determines whether the pixel of interest is a halftone dot candidate pixel, and a pixel of interest that is determined as a halftone dot candidate pixel by the edge density detection unit and the pattern comparison unit. The halftone dot separation apparatus is provided with a correction processing unit that determines whether or not the attribute of the pixel of interest is a halftone pixel based on halftone candidate pixel information of the surrounding area. [Industrial applications] To improve the quality of the reproduced image when playing back an image in which an image represented by halftone dots and a line drawing such as a character are mixed by a facsimile or a copying machine, etc. It is preferable to perform a smoothing process for removing moiré in a region of an image represented by halftone dots, and to perform a sharpening process in a region of a line drawing such as a character. Also, in the case of transmitting an image, the efficiency of data compression can be improved by applying the encoding process according to the characteristics of each area as described above. The present invention relates to a halftone dot region separation method for separating a halftone dot region represented by a halftone dot from another region such as a character region as preprocessing for performing such image processing. [Prior Art] Japanese Patent Application Laid-Open No. 60-80365 discloses that a binarized image of, for example, 5 × 5 pixels including a pixel of interest after binarizing input image data for each pixel with a fixed threshold value. Is compared with a halftone pattern stored in advance, and an area separation method is described in which the number of pixels that match this halftone pattern exceeds a certain value and is detected as a halftone area. [Problems to be Solved by the Invention] However, in such a region separation method, since a fixed threshold value is used for binarization, a halftone dot pattern is formed depending on a halftone dot ratio of a halftone dot region. It is difficult to discriminate, and if the halftone pattern is different from the stored halftone pattern, it may not be detected as a halftone area. SUMMARY OF THE INVENTION It is an object of the present invention to provide a halftone dot area separating apparatus for separating a halftone dot area in an image from other areas such as a character area or a photograph area with high accuracy. [Means for Solving the Problems] An edge density is measured in a local region surrounding a target pixel, and whether or not the target pixel is a halftone dot candidate pixel is determined based on whether or not the edge density exceeds a predetermined threshold value. An edge density detecting unit for determining, and a pattern comparing unit for comparing pixel information of a local area surrounding the target pixel with a predetermined halftone pixel determination pattern to determine whether the target pixel is a halftone candidate pixel If, with respect to the target pixel determined as the halftone candidate pixel by the edge density detecting unit and the pattern comparator unit, attribute dot of the target pixel based on the dot candidate pixel information of the peripheral area of the target pixel A correction processing unit that determines whether a pixel is a pixel or not is provided in the halftone dot region separation device. [Operation] The present inventor measures the edge density in a local region surrounding the pixel of interest, and determines that the attribute of the pixel of interest can be determined as a halftone pixel when the edge density exceeds a predetermined threshold. I found it. In other words, most halftone dot images have the property of having an edge, and the edge density in the local region shown by the bold line surrounding the target pixel indicated by hatching in FIG. For example, it is measured by an edge extraction filter or the like, and when this density exceeds the threshold value TH, the attribute of the target pixel can be determined as a halftone dot or a halftone dot candidate. Although a character has relatively many edges, its edge density is lower than that of a halftone image having a high screen ruling as described above. The determination of the attribute of the pixel of interest based on the edge density and the determination of the attribute of the pixel of interest based on a known pattern comparison are performed independently and in parallel, thereby increasing the accuracy of the attribute determination. By determining whether or not the attribute of the target pixel is a halftone pixel based on halftone candidate pixel information of the surrounding area, the halftone area can be separated from other character areas with extremely high accuracy. it can. [Embodiment] FIG. 1 shows a principle embodiment of the present invention. A block surrounded by a dashed line and indicated by 10 is an edge density detection unit, and a block surrounded by a dashed line and indicated by 20 is a pattern comparison unit. And 40, a correction processing unit that performs correction based on the peripheral information. When the edge density detecting section 10 and the pattern comparing section 20 determine that the target pixel is a halftone dot candidate, they send an “H” level output to the OR circuit 30. Is determined to be a halftone dot candidate, the output of the “H” level indicating the halftone dot candidate is sent from the OR circuit 30 to the correction processing unit 40, and the output is determined based on the halftone dot determination information on the peripheral pixels. Correction is performed. More specifically, in the edge density detecting means 11 of the edge density detecting unit 10, filtering is performed on all pixels in a local region surrounding the pixel of interest by an edge extraction filter shown by a thick frame in FIG. If the number of pixels determined to exceed the predetermined value TH, the target pixel is determined to be an edge. FIG. 3 shows an example of an edge filter indicated by a bold line in FIG.
× 3 pixels = multiply the respective density values of the nine pixels by the coefficient shown in the figure, add the obtained pixel values, and obtain the edge value of the pixel located at the center,
If the edge value exceeds a predetermined value, it is determined that the pixel is an edge pixel. The block 20 is for determining a halftone pixel by a known pattern matching. In the illustrated embodiment, the edge emphasizing process 21 is performed, then the binarization 22 is performed, and a halftone pattern and a previously prepared halftone pattern are determined. By performing the matching 23, it is determined whether or not the pixel of interest is a halftone dot candidate.
The determination result of the “level” is supplied to the other input terminal of the OR circuit 30. Therefore, the OR circuit 30 outputs a pixel determined to be a halftone candidate by at least one of the edge density detection unit 10 and the pattern detection unit 20. Is output at the “H” level, and this output is supplied to the correction processing unit 40. In the correction processing unit 40, the determination result as the halftone dot candidate is determined by the halftone dot determination information on the pixels around the pixel of interest. Finally, it is determined whether or not a pixel is a halftone pixel based on the following equation. In the case of a halftone dot, based on the high probability that the pixel of interest is also a pixel belonging to a halftone dot area, as indicated by hatching, k × k around the pixel of interest determined to be a halftone dot candidate pixel Pixel If the area includes a pixel determined to be a halftone dot candidate pixel, the pixel of interest is corrected to a pixel belonging to the halftone area, thereby making the halftone area highly accurate with respect to other character areas and photographs. [Effect of the Invention] In the present invention, the process of determining a halftone dot region by the edge density detection unit does not depend on the stored pattern, and Since it is excellent in the determination of a halftone dot region having a high screen ruling, it is possible to compensate for the weakness of the halftone dot region determination processing by the pattern comparing unit. A special effect is obtained in that the halftone dot region can be separated from the character region, the photograph region, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a principle embodiment of the present invention, FIG. 2 is a diagram showing a principle of edge density measurement, FIG. 3 is an edge filter used in the embodiment FIG. 4 is a diagram for explaining correction based on halftone dot determination information of a peripheral area in the embodiment.

Claims (1)

Claims: 1. An edge density is measured in a local region surrounding a target pixel, and whether the target pixel is a halftone dot candidate pixel is determined based on whether the edge density exceeds a predetermined threshold. An edge density detection unit that determines whether the pixel of interest is a dot candidate pixel by comparing the pixel information of the local area surrounding the pixel of interest with a predetermined dot pixel determination pattern A comparing unit, for the pixel of interest determined to be a halftone dot candidate pixel by the edge density detection unit and the pattern comparison unit, the attribute of the pixel of interest based on halftone candidate pixel information of a peripheral region of the pixel of interest; A correction processing unit that determines whether or not a pixel is a halftone pixel .